This invention relates to a method of producing an image of an area under investigation, in which method wave pulses generated by a pulsed source of coherent waves are transmitted into the area, the echo waves reflected from reflectors in the area are received, and a corresponding electrical echo signal is generated in each case. The invention also relates to apparatus for performing the method.
Amongst methods of the above kind, ultrasound imaging of organs is, for example, increasingly used in medical diagnosis. The advantages of ultrasound imaging over X-ray methods are that non-ionizing radiation is used, that soft tissue can be imaged without contrast agents, that real-time imaging is possible, and that ultrasound equipment is available at a relatively low cost.
Medical ultrasound diagnosis has today reached a level at which widespread clinical use is possible. Image quality, however, is impaired, inter alia, by speckle noise which is marked by a granular appearance of the ultrasound images and which is an obstacle to tissue differentiation. Small structures or interfaces met at an unfavourable angle are unrecognizable because of the image speckle, i.e. a granular structure of an ultrasound image. This is an artefact produced by the coherence of the ultrasound. Speckle is today the main obstacle to the recognition of details in ultrasound images. Because of speckle, the signal-noise ratio of an ultrasound image has a value of only 1.91 (C. B. Burckhardt, "Speckle in Ultrasound B-Mode Scans", IEEE Trans. on Sonics and Ultrasonics, vol. SU-25, 1978 pages 1-6). Speckle thus has an adverse effect on the recognition of small and/or low-contrast structures. e.g. metastases. Various methods therefore have already been proposed in order to reduce speckle but they all have certain disadvantages. The methods used to date and their disadvantages are as follows:
1. Low-pass filtering of image signals
In this method the amplitude of the image signals is averaged over a number of speckle grains, so that the fluctuations of such amplitudes are reduced. This method is hardly used because resolution is reduced as a result.
2. Averaging of the image signals over a plurality of images recorded at different frequencies
This is known as the frequency compounding method (P. A. Magnin, O. T. von Ramm, F. L. Thurstone, "Frequency Compounding for Speckle Contrast Reductions in Phased Array Images", Ultrasonic Imaging, vol. 4, 1982, pages 267-281). Since the images are recorded at different frequencies, the speckle in the different images is substantially uncorrelated in this method (depending upon the overlapping of the frequency bands) and the averaged image shows less speckle. However, this method has the following disadvantages:
A wide-band system is required to record images at a plurality of frequencies. Since the ultrasound absorption in the tissue increases with increasing frequency, the number of usuable frequencies is theoretically limited.
The wide-band system could be used to give better longitudinal resolution. Division into a plurality of frequency bands gives a longitudinal resolution less than the maximum possible resolution.
3. Averaging of the image signals over a plurality of images recorded from different directions
This is the principle of the compound scan method (C. B. Burckhardt, "Speckle in Ultrasound B-Mode Scans", IEEE Trans. on Sonics and Ultrasonics, vol. SU-25, 1978, pages 1-6; D. P. Shattuck and O. T. von Ramm, Ultrasonic Imaging 4, 1982, pages 93-107). Although this method gives a considerable reduction of speckle and hence a corresponding improvement of image quality it has the following disadvantages:
The complexity of a device for using this method is much greater than the complexity of a normal ultrasound device.
More time is required to record images in different directions, and hence the maximum possible image rate drops.
The structure to be shown must be "visible" to ultrasound over a relatively large angle range, i.e. a larger "ultrasound window" is required than with conventional methods.
Ultrasound imaging is not the only imaging method confronted by the speckle problem. Similar imaging methods operating with coherent radiation, e.g. radar, are also concerned by the problem. Specific mention may be made of "Synthetic Aperture Radar" (W. M. Brown, J. L. Porcello, "An Introduction to Synthetic Aperture Radar". IEEE Spectrum vol. 6, 1969, pages 52-62).